The use of optical fiber for long-distance transmission of voice and/or data is now common. As the demand for data carrying capacity continues to increase, there is a continuing need to utilize the bandwidth of existing fiber-optic cable more efficiently. An established method for increasing the carrying capacity of existing fiber cable is Wavelength Division Multiplexing (WDM). In this method, multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light and each light-wave-propagated information channel corresponds to light within a specific wavelength range or “band.”
In this specification, these individual information-carrying lights are referred to as either “signals” or “channels.” The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a “composite optical signal.”
Because of the increased network traffic resulting from the use of the WDM technique, there is an increasing need for optical switching and routing devices that can quickly route or re-route numerous channels amongst various optical communications lines. An optical interferometer is a device that produces a periodic phase modulation in a composite optical signal. When an optical interferometer is incorporated as a component within another optical apparatus, this periodic phase modulation may be utilized advantageously to produce periodic transmission, reflection, optical delay or polarization properties within optical signals or composite optical signals passed through the apparatus. Moreover, if the optical interferometer is tunable, the peak positions of the modulated functions may be controllably varied so as to align the peak positions to standard channel positions to maximize optical throughput or so as to switch or route channels amongst various outputs.
Various mechanical, thermo-optic, electro-optic or magneto-optic methods have been employed to provide tuning capabilities to optical interferometers. Although these methods provide adequate tuning capabilities, they invariably add additional optical components and/or electronic connections to the interferometer, thereby increasing the complexity and difficulty of fabricating and aligning the interferometer and potentially reducing the stability and optical throughput of the interferometer. Accordingly, there remains a need for an improved tunable interferometer that does not incorporate additional elements into the optical path within the interferometer and that does not disturb or move any of the optical components disposed within or associated with this optical path. The present invention addresses such a need.